Whisk together the flour, eggs, milk, sugar, salt, and 4 1/2 teaspoons of the butter to form a smooth, thin batter. Refrigerate for at least 1 hour before proceeding.
Heat a heavy 6-inch skillet or crepe pan over medium-high heat. When hot, brush with a light coating of the remaining butter. Ladle about 1/4 cup of crepe batter into the pan, tilting the skillet to evenly coat the pan with batter. Cook until golden brown on the bottom and the top begins to look dry, 1 to 2 minutes. Using a spatula, carefully turn the crepe and cook on the second side just until the bottom colors slightly, about 30 seconds. Transfer to a plate and cover loosely to keep warm. Repeat with remaining batter.
Assemble as directed in Blueberry-Mascarpone Crepes recipe, and serve. (Or, make ahead and when cool, layer between sheets of waxed paper and wrap tightly in plastic wrap. Refrigerate until ready to use, up to 3 days.)
Yield: 12 (6-inch) crepes